8.10   Inspecting Venting Systems

Combustion gases are vented through vertical chimneys or other types of approved horizontal or vertical vent piping. Iden­tifying the type of existing venting material, verifying the cor­rect size of vent piping, and making sure the venting conforms to the applicable codes are important tasks in inspecting and repairing venting systems. Too large a vent often leads to con­densation and corrosion. Too small a vent can result in spillage. The wrong vent materials can corrode or deteriorate from heat.

NFPA Codes

The National Fire Protection Association (NFPA) publishes authoritative information on material-choice, sizing, and clear­ances for chimneys and vent connectors, as well as for combus­tion air. The information in this venting section is based on the following NFPA documents.

•       NFPA 54: The National Fuel Gas Code

•       NFPA 31: Standard for the Installation of Oil-Burning Equipment

•       NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid-Fuel-Burning Appliances

8.10.1   Vent Connectors

A vent connector connects the wood stove’s vent collar with the chimney. Approved vent connectors for gas-fired units are made from the following materials.

•       Type-B vent, consisting of a galvanized steel outer pipe and aluminum inner pipe for gas-fired units.

•       Type-L vent connector with a stainless-steel inner pipe and a galvanized-steel outer pipe for oil-fired units.

•       Double-wall stove-pipe vent connector with a stainless-steel inner pipe and a black-steel outer pipe for solid-fuel units.

•       Galvanized steel pipe for gas or oil-fired units only: See table.

Double-wall vent connectors are the best option, especially for appliances with some non-vertical vent piping. A double-wall vent connector maintains a sufficient flue gas temperature and prevents condensation.

Anywhere that draft is weak or flue gas temperature is low, use a double-wall vent connector. Gas appliances with draft hoods installed in attics or crawl spaces must use a Type-B vent con­nector. Use Type-L double-wall vent pipe for oil vent connectors in attics and crawl spaces.

Vent-Connector Requirements

Verify that vent connectors comply with these specifications.

•       Vent connectors must be as large as the vent collar on the appliances they vent.

•       Single wall vent-pipe sections must be fastened together with 3 screws or rivets at each joint.

•       Vent connectors must be sealed tightly where they enter masonry chimneys.

•       Vent connectors must be free of rust, corrosion, and holes.

•       Maintain minimum clearances between vent connectors and combustibles.

•       The chimney combining two draft-hood vent connectors must have a cross-sectional area equal to the area of the larger vent connector plus half the area of the smaller vent connector. This common vent must be no larger than 7 times the area of the smallest vent connector. For specific vent sizes, see the NFPA codes listed on page 307. 

•       The horizontal length of vent connectors shouldn’t be more than 75% of the chimney’s vertical height or have more than 18 inches horizontal run per inch of vent diameter.

•       Vent connectors must have upward slope to their connec­tion with the chimney. NFPA 54 requires a slope of at least ¹/₄-inch of rise per foot of horizontal run so that combus­tion gases rise through the vent. The slope also prevents condensation from collecting in the vent and corroding it.

•       When two vent connectors connect to a single chimney, the vent connector servicing the smaller appliance must enter the chimney above the vent for the larger appliance.